1. Field of the Invention
This invention relates to an automatic focusing apparatus for camera, such as a video camera, an electronic camera, and so on.
2. Description of the Related Art
An apparatus for automatically focusing on a subject is disclosed in Japanese patent application number 4-6413 (Publication number 5-191708 published Jul. 30, 1993).
In this related art, a process circuit extracts a luminance signal from a picture signal picked up by a CCD via a lens system, and a contrast detection section detects a contrast of each image from the luminance signal. Then a focus drive circuit forwards a position of a lens system in a close end direction by a movement in response to a depth of field and when the contrast is larger than a value at a point before the movement, the circuit retracts the lens system position in the infinite end direction and uses a point at which a focal point is placed within a range of a depth of field as a focal position by the lens system.
However, a problem encountered with this prior art apparatus is that the focus drive circuit has to forward and retract a position of a lens system repeatedly. In addition, the focus drive circuit reduces with predetermined fixation ratio the range of the movement of the lens system gradually in order to place the focal point within a range of a depth of field. Consequently, the apparatus needs a lot of time until the focal point is placed within a range of a depth of field as a focal position by the lens system. Therefore, the apparatus can not focus on a subject quickly.
Accordingly, it is an object of the present invention to focus on a subject quickly by omitting a waste of a lens system""s operation.
Another object of the invention is to stop the lens system at the closest possible position to a focal point.
Still another object of the invention is to provide an apparatus that an external device receiving an image data does not need to distinguish whether the image data is in focus or not.
The present invention is an automatic focusing apparatus for automatically focusing on a subject in an image plane. The automatic focusing apparatus includes a focus lens for focusing on the subject, drive means for moving the focus lens, an image element for picking up image data from the subject through the focus lens, an image processor for extracting luminance signals of each pixel from the image data picked up by the image element, and a controller for computing contrast value under the luminance signals of each pixel extracted by the image processor, for computing moving quantity of the focus lens by using inverse number of the contrast value, and for controlling the drive means according to the moving quantity.